The invention relates to a device for storing digital information in the form of magnetic domains. Such a device is known and comprises a control device which is connected to an information input, a nonmagnetic substrate layer, a first layer of magnetic material which is structurally integral therewith and in which magnetic domains can be sustained by a field of a bias magnetic fluid generator which field is transverse to the layer, a second layer of magnetic material which is structurally integral with the said first layer and in which further magnetic domains can be sustained by the field of the bias magnetic field generator, first means for successively generating a first series of magnetic domains in the first layer, and second means for successively generating a second series of magnetic domains in the second layer. The device is adapted to form, under the selective control of the control device and by cooperation by the first and second means, both single and superposed magnetic domain pairs in the first and second layers of magnetic material. Superposed magnetic domain pairs comprise a magnetic compensation wall at their interface. All non-superposed magnetic domains and superposed magnetic domain pairs always exhibit mutually repellent forces, including a force component in the plane of the layers. Each of the first and second layers of magnetic material comprise at least two magnetic crystalline sublattices with first and second magnetizations which are complementary within a layer and which have substantially opposed directions. The device further comprises propagation means for producing a relative displacement between the first and second means and the generated series of magnetic domains or magnetic domain pairs. It also comprises a detector unit for the separate detection of single magnetic domains and superposed magnetic domain pairs, and also an annihilation unit for destruction of magnetic domains and magnetic domain pairs.
Magnetic domains having a substantially circular shape are known as "bubbles". The above known device is described in the article "Observation of various types of bubbles bounded by a compensation wall", by J. Haisma et al, IEEE Transactions on Magnetics, volume MAG 10, 630. Magnetic bubbles are generally highly regarded, because they allow a high information density and because a constant supply of energy is not required for sustaining the information. The domains can be easily generated and displaced in a desired direction in the magnetic material. Domains of this kind can also be easily detected, inter alia by way of faraday rotation. Known devices which utilize domains containing information usually require a long information access time, because serial operation is required. The article discloses the possibility of providing pairs of superposed domains which enables optical detection in parallel.